On a standard five prong dual pole 20 or 30 amp Bosch relay, the terminals are as follows:

30 goes to a power source such as the battery or junction box on the fender near the battery (On a 240 Turbo at least).

85 is either the ground or the trigger. I always get this and 86 mixed up. Look at a few under the hood to determine which is ground. The ground goes to a good ground point. This is the relay ground and not the ground for the device the relay will be providing power for.

86 is either the trigger or the ground. The trigger just turns the relay on. The wire need not be as heavy guage as the wires for power,ground, and power to the device the relay will be providing power for. You can turn the relay on with an unswitched power source, with a switched power source, with a boost onset triggered device, or otherwise. Switched means it has power only when the ignition is on. There are two versions of switched. Switched to key position I and switched to key position II. You might not need to get that far. Just find something that has power when the engine is running but does not have power when the engine is off and use that ofr switched. The ignition coil positive side can spare the power to trigger a relay (though not to run a device). Unswitched means it is on all the time. An unswitched power source would be the battery or the junction box next to the battery.

I use a boost referenced switch in the trigger wire to turn my electric primary fan on under turbo boost. So the trigger wire from the coil positive side goes to a boost pressure switch that opens/closes at a certain boost level and then on to the relay. That way, the relay is switched on and provides power to the fan when the turbo pumps a certain amount of boost.

I have another wire going to the relay trigger from the electric fan thermostat. The other thermostat terminal connects to another switched power source.

87 provides power to the device. Use appropriate guage wire. Generally the device grounds near where it is located.

87a is present on dual pole relays. Dual pole means the relay switches from 87a to 87 when triggered. Thus 87a will always provide power to the device, but when the relay is triggered, 87a will switch off and 87 will switch on. I am sure you can think of some creative ways to use this feature.

There are many other relays. Some are designed to have multiple triggers or control multiple devices. Others have delay timers inside them.

The best thing is to experiment a bit before you do your final wire installation. Try to make sure the relay is doing what you want when you want.

Philip Bradley

87 is